Boron-containing preceramic blend and fiber formed therefrom

ABSTRACT

A process for producing boron-containing ceramics such as boron carbide and boron nitride comprises pyrolyzing a blend of a precarbonaceous polymer such as polyacrylonitrile and a boron-containing polymer such as that formed by the reaction of a borane with a Lewis base. Pyrolyzation in an inert atmosphere yields boron carbide while pyrolyzation in a reactive gas burns away the precarbonaceous polymer and yields a ceramic comprising the reaction product of boron and the pyrolyzation gas. Boron nitride ceramics are formed by pyrolyzing the preceramic blend in ammonia.

This application is a division of application Ser. No. 082,761, filedAugust 7, 1987, now U.S. Pat. No. 4,832,895. Which is aContinuation-In-Part application of U.S. Ser. No. 933,413, filedNovember 21, 1986 now abandoned.

FIELD OF THE INVENTION

The present invention is directed to the formation of boron-containingfibers. More specifically, the present invention is directed to a methodof forming boron carbide and boron nitride fibers from boron-containingpolymers.

BACKGROUND OF THE INVENTION

In the search for high performance materials, considerable interest hasbeen focused upon carbon fibers. The terms "carbon" fibers or"carbonaceous" fibers are used herein in the generic sense and includegraphite fibers as well as amorphous carbon fibers. Graphite fibers aredefined herein as fibers which consist essentially of carbon and have apredominant x-ray diffraction pattern characteristic of graphite.Amorphous carbon fibers, on the other hand are defined as fibers inwhich the bulk of the fiber weight can be attributed to carbon and whichexhibit an essentially amorphous x-ray diffraction pattern.

Industrial high performance materials of the future are projected tomake substantial utilization of fiber reinforced composites, and carbonfibers theoretically have among the best properties of any fiber for useas high strength reinforcement. Among these desirable properties arecorrosion and high temperature resistance, low density, high tensilestrength and high modulus. During such service, the carbon fiberscommonly are positioned within the continuous phase of a resinous matrix(e.g. a solid cured epoxy resin). Uses for carbon fiber reinforcedcomposites include aerospace structural components, rocket motorcasings, deep-submergence vessels, ablative materials for heat shieldson re-entry vehicles, strong lightweight sports equipment, etc.

As is well known in the art, numerous processes have heretofore beenproposed for the thermal conversion of organic polymeric fibrousmaterials (e.g. an acrylic multifilamentary tow) to a carbonaceous formwhile retaining the original fibrous configuration substantially intact.During commonly practiced carbon fiber formation techniques, amultifilamentary tow of substantially parallel or columnized carbonfibers is formed with the individual "rod-like" fibers lying in aclosely disposed side-by-side relationship. See for instance, thefollowing commonly assigned U.S. Pat. No. 3,539,295; 3,656,904;3,723,157; 3,723,605; 3,775,520; 3,818,082; 3,844,822; 3,900,556;3,914,393; 3,925,524; 3,954,950; and 4,020,273.

Recently, there has been interest in the use of ceramic materials,including ceramic fibers for a number of high temperature, highperformance application such as gas turbines. These applications requirea unique combination of properties such as high specific strength, hightemperature mechanical property retention, low thermal and electricalconductivity, hardness and wear resistance, and chemical inertness.

Among the ceramic materials which have been suggested are those madefrom organosilicon polymers. Thus, polymers based on silicon, carbonand/or nitrogen and oxygen have been developed. See, for example,"Siloxanes, Silanes and Silazanes and the Preparation of Ceramics andGlasses" by Wills et al, and "Special Heat-Resisting Materials fromOrganometallic Polymers" by Yajima, in Ceramic Bulletin, Vol. 62, No. 8,pp. 893-915 (1983), and the references cited therein.

Other metallic polymers can be formed into ceramic materials. Thus, U.S.4,581,461 forms boron nitride by pyrolyzing B-triamino-N-tris(trialkylsilyl)borazines. The formation of aluminum nitride fibers isdisclosed in commonly assigned, copending application Ser. No. 872,312,filed June 9, 1986. Aluminum nitride ceramics are formed by thermalconversion of poly-N-alkyliminoalanes. Ceramics comprising siliconcarbide and aluminum nitride solid solutions are also disclosed. Theseceramic alloys are formed by thermal conversion of a mixture of anorganosilicon preceramic polymer and the above-mentionedaluminum-containing polymer. Moreover, many recent patents describespecific silicon-containing preceramic polymers which are formed intosilicon carbide and/or nitride upon thermal treatment.

Another technique which has been suggested for producing ceramic fiberssuch as metal carbide fibers has involved incorporating metallicadditives into a carbon fiber product, the precarbonaceous polymerforming solution, the polymer spinning solution or the polymer fibersubsequent to spinning, and converting the metallic compounds in situ tometal carbides upon thermal conversion. In these methods theprecarbonaceous polymer acts as the source of carbon.

An important ceramic fiber formed by such method is boron carbide andboron carbide-containing carbon fibers. The addition of boron carbide tocarbon fiber is known to increase fiber strength and, more particularly,to increase the oxidative stability of carbon fibers such that the boroncarbide-containing carbon fibers can withstand high temperatureenvironments. Methods of incorporating boron into carbon fibers to formboron carbide fibers have typically involved treating the carbon fiberswith gaseous boron halides or impregnation with boric oxides includingboric acid, metallic borates and organic borates including alkyl andaryl borates. Upon being treated with the boron compounds, the fibersare heated to initiate reaction of boron with the carbon fibers to yieldboron carbide.

For producing an ideal boron carbide fiber, boron levels in the fibersmuch reach about 78 wt. %. Unfortunately, prior processes for producingboron carbide fibers such as treating carbon fibers with boric acid havetypically yielded only small boron loadings, e.g., 2-5 wt. %. Such smallboron loadings do not result in any appreciable improvement in theoxidative stability of the loaded carbon fibers at elevatedtemperatures.

Examples of U.S. patents which disclose incorporating boron into carbonfibers and other shaped articles are discussed below.

U.S. Pat. No. 3,399,979 discloses a method of forming nitride articlesby a process which involves impregnating a preformed organic polymericmaterial with a metal-containing compound, heating the impregnatedpolymer to carbonize the polymer and heating in an atmosphere containingnitrogen-containing compounds such as ammonia to produce the metalnitride. More sepcifically, a method is disclosed of immersing a rayonyarn in an aqueous solution of ammonium decaborane and treating theimpregnated yarn in ammonia to form boron nitride.

U.S. Pat. No. 3,403,008 discloses a process for producing metal carbidefibers and the like which comprises treating an organic polymeric fiberwith a solution containing a metallic compound, heating the metalcompound-imbibed polymer to form the carbonaceous fiber and furtherheating the fiber in a nonoxidizing atmosphere to react the metal withthe carbonaceous fiber to form a metal carbide. Among the metal carbideswhich can be formed is boron carbide obtained by treating the organicfiber with a boric acid solution.

U.S. Pat. No. 3,672,936 discloses providing a reinforced carbon orgraphite article having improved oxidation resistance and increasedstrength by incorporating therein the in-situ reaction product of carbonand a boron containing additive. The process involves making a carbonarticle such as carbon fiber, dispersing the boron containing additivewith at least a portion of the carbon fiber, impregnating the carbonfiber with a carbonizable binder, and heating the fiber to carbonize thebinder and to form in-situ the reaction product of carbon and the boroncontaining additive. Boron containing additives include metal borides,boron nitride, or boron silicides as well as elemental boron.

U.S. Pat. No. 3,971,840 discloses a process of forming a carbidecontaining fiber of improved strength by a process of heating acarbonaceous fiber in the vapor of a halide of a carbide forming elementand heating the fiber under a controlled degree of tension to result inthe formation of the carbide. A boron carbide fiber can be formed bytreating a carbon fiber with boron trichloride.

U.S. Pat. No. 4,010,233 discloses a method of producing an inorganicfiber comprising a metal oxide phase and finely divided dispersed phase.Boron compounds such as boranes can be used to provide the dispersedphase in the fibers.

U.S. Pat. No. 4,097,294 suggests that a boron carbide ceramic isobtainable from a carborane carbon polymer and that a boron nitrideceramic is obtainable from a borazene polymer. A mixed ternary ceramicis obtained from a polymer with a repeating unit of [C₂ B₁₀ H₁₀ R₂ Si(R₂SiO)_(n) ]_(x), wherein n is 1 to 10 and x is greater than 4.

U.S. Pat. No. 4,126,652 discloses a method of forming metal carbidearticles especially fibers by reacting a metal powder with a carbonfiber such as has been formed from polyacrylonitrile. The fiber may beproduced by subjecting a monomer mixture mainly comprising acrylonitrileto solution polymerization with addition of the powdery metal prior to,in the course of, or after the polymerization so as to disperse themetal powder into the polymerization mixture, or alternativelydispersing the powdery metal into a solution of the acrylonitrilepolymer in a suitable solvent, and then subjecting the thus obtainedpowdery metal-containing mixture to molding by a conventional dry or wetmethod. An example of forming a boron carbide fiber comprises mixingdimethylformamide, non-crystalline boron and polyacrylonitrile to obtaina viscous dispersion and spinning the viscous liquid into fiber which isthen heated up to 1800° C. to form the boron carbide.

U.S. Pat. No. 4,197,279 discloses an acrylic carbon fiber with excellentthermal oxidation resistance which contains a phosphorus componentand/or a boron component as well as a zinc and/or calcium component. Thecarbon fibers can be produced by incorporating the above components intothe fibers including the acrylic fibers or into the carbon fibers whichare produced. For example, the above described compounds can be added tothe reaction mixture to produce the acrylic polymer or to a solution ofthe acrylic polymer before spinning into the fibers. Compounds are addedin desired amounts as an aqueous or organic solution or dispersionthereof. Suitable boron compounds which can be used include boric acids,boric acid salts of metals and boric esters such as alkyl borates. Theamount of boron included in the fibers is measured in parts per millionwith amounts as high as only 5100 ppm being described. Similar to thispatent is U.S. Pat. No. 4,412,937 which discloses adding 0.01 to 0.03%by weight boron onto a carbon fiber formed from acrylic polymer.

U.S. Pat. No. 4,424,145 discloses a carbon fiber derived from mesophasepitch which has been boronated and intercalated with calcium so that thefiber contains from about 0.1% by weight to about 10% by weight boronand the calcium to boron weight ratio in the fiber is 2:1. Theboronating step can be carried out with elemental boron, BCl₃, boranes,or water soluble compounds of boron such as boric acid.

There still exists in the art a need to provide an improved process forproducing boron-containing fibers. Thus, there is a need to form boroncarbide-containing fibers which have improved oxidation resistancerelative to carbon fibers such that the fibers can find increased use inthe high temperature, high performance application for which such fibershave vast potential. There is a need to produce improved boron carbidefibers which contain boron in amounts greater than what has heretoforebeen achieved in the art. Moreover, boron nitride has use as anelectrical insulator, a neutron insulator and is corrosion resistant andis, thus, a very useful ceramic which would be advantageous in fiberform.

Accordingly, a principle object of the present invention is to provideboron-containing fiber with increased boron content.

Another object of the invention is to provide boron carbide fibers whichhave increased oxidative stability at elevated temperatures relative tocarbon fibers.

Another object of the present invention is to provide improved boroncarbide fibers and an improved process for forming same.

Yet another object of the present invention is to provide improved boronnitride fibers and to an improved process for forming same.

These and other objects and advantages of the invention will be apparentupon consideration of the following description of the embodiments setforth in the description of the invention and the appended claims.

SUMMARY OF THE INVENTION

In accordance with the present invention boron-containing fibers areprovided by forming a blend of a boron-containing polymer and aprecarbonaceous polymer, shaping the blend into a fiber such as byspinning, and pyrolyzing to form a fiber comprising a boron ceramic. Theparticular boron ceramic which is formed will depend upon thepyrolyzation atmosphere. Thus, pyrolyzing the spun fiber in an inert gaswill yield a boron carbide fiber whereas pyrolyzing in ammonia will leadto a boron nitride fiber. Other pyrolyzation atmospheres such asphosphine or metalloid-containing gases will lead to the formation of aboron phosphide, etc. By precarbonaceous polymer is meant any polymericmaterial which can be converted into carbon at elevated temperatures.The boron-containing polymers such as a polymeric borane used in thepresent invention can, for example, be formed by condensation of aborane with a Lewis base and are typically of low molecular weight,poorly characterized and are difficult to spin into fibers. However, byforming a spinning composition comprising a blend of a precarbonaceouspolymer such as polyacrylonitrile with the boron-containing polymer, aboron loaded fiber can be produced which has substantially greater boronloadings than achieved by the impregnation methods of the prior art andin view of the polymeric nature of the boron component, can be morereadily spun into fibers than methods employing inorganic materials asthe boron source.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The precarbonaceous polymers to be used in the present invention arethose polymeric materials whether natural pitches or the like orsynthetic polymers which can be molded, preferably spun into fibers, andcan be carbonized to yield a carbon article or burned off completely. Aparticularly preferred polymer is polyacrylonitrile (PAN) and copolymersthereof which are used in the formation of carbon fibers. Thus, polymerscontaining not less than about 80% by weight of units of acrylonitrileare favorable. When the acrylonitrile polymer is a copolymer, the othermonomeric units may be derived from any monomer copolymerizable withacrylonitrile of which preferred examples are acrylic acid and estersthereof such as methyl acrylate, ethyl acrylate, 2-chloroethyl acrylate,2-hydroxy-3-chloropropyl acrylate, 2,3-dibromopropyl acrylate,tribromophenyl acrylate, 2-hydroxyethyl acrylate, 2-methoxyethylacrylate, methoxypolyoxyethylene acrylate and N,N-dimethylaminoethylacrylate, methacrylic acid and esters thereof corresponding to the abovementioned acrylic acid esters, derivatives of acrylic acid esters suchas methyl 2-hydroxymethylacrylate and methyl2-hydroxymethylmethacrylate, itaconic acid and ester derivativesthereof, allylamine and derivatives, diallylamine and derivatives,phosphorus-containing monomers such as dimethyl2-cyano-1-methylallylphosphonate, dimethyl 2-cyano-allylphosphonate anddimethyl 2-ethoxycarbonyl allylphosphonate, styrene and derivatives suchas sodium p-styrenesulfonate, chloromethylstyrene and 1-methylstyrene,vinyl acetate, acrylamide, dimethylacrylamide, diacetacrylamide, methylvinyl ketone, methyl isopropenyl ketone, methacrylonitrile, vinylidenecyanide, 1-cyanovinyl acetate, 2-hydroxymethylacrylonitrile,2-acetylaminomethylacrylonitrile, 2-methoxymethylacrylonitrile,2-(1-hydroxyethyl)acrylonitrile, 2-ethoxymethylacrylonitrile vinylidenechloride, vinyl bromide, sodium allylsulfonate, sodiummethallylsulfonate, allyl alcohol, methallyl alcohol, etc.

The molecular weight of the acrylonitrile polymer is usually in a rangeof about 30,000 to 300,000. More particularly, it may be preferablychosen in such a manner that the viscosity at the molding step becomesfrom about 50 to 10,000 poise.

Other suitable precarbonaceous polymers include polyisobutylene,polyisoprene, polystyrene, polymethyl methacrylate, polyvinyl alcohol,polyacrylamide, polyacrylic acid, polyethylene oxide, cellulose,carboxymethyl cellulose, hydrolyzed starch, dextran, guar gum,polyvinylpyrrolidone, polyurethane, polyvinyl acetate, and the like, andmixtures thereof.

As the boron source to be included in the mixture, borane-containingpolymers are preferably used. Preferably, the borane-containing polymersare prepared by the condensation of boranes with Lewis bases. Suchpolymers are well known and prepared by condensing a borane such asdiborane, pentaborane or decarborane with Lewis bases such as amines,amides, isocyanates, nitriles and phosphines. Any borane containing 2-10boron atoms are useful. A particularly preferred borane-containingpolymer is one formed by the condensation of decarborane anddimethylformamide (DMF). Such polymer has a high boron content, andsince DMF is a typical solvent for PAN, polymerization and blending ofthe polymeric materials can be achieved in one step. The borane-Lewisbase condensation polymers are known and described, for example, inPOLYMER LETTERS, Vol. 2, pp. 987-989 (1964); Chemical Society (London)Spec. Publ. No. 15 (1961), "Types of Polymer Combination among theNon-metallic Elements", Anton B. Burg, pp. 17-31; U.S. Pat. Nos.2,925,440; 3,025,326; 3,035,949; 3,071,552; and British Patent 912,530,all of which are herein incorporated by reference. The borane-containingpolymers are characterized by a low molecular weight, typically notexceeding 20,000, and more typically, ranging from about 400 to 5,000,and often between 400 and 2,000. The polymers which are formed arepoorly characterized, some of which are linear and others of which arebranched structures, are brittle and, thus, break apart readily and arenot very stable. Accordingly, these borane-Lewis base condensationpolymers are not easily shaped such as being spun into fibers.

Other borane-containing polymers can be used including those disclosedin U.S. Pat. No. 3,441,389 herein incorporated by reference. In thismentioned patent, borane polymers are prepared by heating a compound ofthe formula (RAH₃)₂ B₁₀ H₁₀ or (RAH₃)₂ B₁₂ H₁₂ at a temperature of200°-400° C. for several hours. Moreover, borazines such as disclosed inU.S. Pat. No. 4,581,468 and carborane polymers such as suggested in U.S.Pat. No. 4,097,294 can also be cospun with the precarbonaceous polymerof this invention.

The boron-containing polymer and precarbonaceous polymer are blended toyield an article-forming mixture which mixture can be shaped by variousmethods known in the art. Thus, the blends of the present invention canbe formed into sheets, films, or articles by molding, extrusion, diepressing, etc. The blends of this invention are more particularly spuninto fibers by known spinning techniques. The relative proportions ofthe boron-containing polymer to precarbonaceous polymer can vary widelyand will depend upon the article being formed and method of forming.Broadly, weight ratios of boron-containing polymer and precarbonaceouspolymer can vary between 9:1 and 1:9. For the production of boronceramic fibers, the weight ratio will preferably be more narrow with theamount of boron-containing polymer to precarbonaceous polymer rangingfrom a ratio of about 5:1 to 1:5, with weight ratios of 5:1 to 1:1 beingmost preferred.

The weight ratio of the boron-containing polymer to precarbonaceouspolymer will also vary widely depending upon the particular boronceramic which is desired. Thus, to form wholly boron carbide (B₄ C)articles requires that the boron content in the blend be substantiallygreater than the carbon content provided by the precarbonaceous polymer.Accordingly, in such instance, it would be desirable to provide a blendwhich contained the minimum amount of precarbonaceous polymer as thecarbon source. However, if the blend is to be shaped into fibers such asby spinning there may be a limit as to the amount of boron-containingpolymer which can be included in the spinning blend inasmuch as theboron-containing polymers are of relatively low molecular weight anddifficult to spin into fibers. On the other hand, if a boron nitrideceramic is to be formed, a low amount of the boron-polymer in the blendis not a disadvantage inasmuch as the precarbonaceous polymer is burnedaway during the pyrolyzation stage. The minimum amount ofboron-containing polymer needed to form a boron nitride ceramic would besuch amount as needed to form an intact boron nitride chain so as tomaintain fiber integrity.

The present invention is particularly useful in the formation of boronceramic fibers from a spinning composition comprising the blend ofboron-containing polymer and precarbonaceous polymer. While it may bepossible to melt spin the blend of the present invention, most likelythe boron polymer will have a melting point far above the melting pointof the precarbonaceous polymer which may be adversely effected at thetemperatures required for melt spinning the boron-polymer. Accordingly,a solvent spinning method is preferred. Thus, spinning into fibers ispreferably accomplished with either the wet or dry spinning techniques.In dry spinning, the spinning composition issues from the spinningapparatus through a spinning column wherein a stream of drying gas issimultaneously fed through the spinning column. The temperature of thespinning column and that of the drying gas is dependent on the volatileswhich have to be evaporated from the filament during its passage throughthe spinning column. In wet spinning, the spinning dope is extruded intoa spin bath where coagulation of the spinning solution and the formationof the fiber occurs. A variety of suitable solvent-nonsolvent systemsare known in the fiber art for use as the coagulating medium or spinbath. Suitable spin baths are nonsolvents for the polymers contained inthe spinning blend and do not chemically react with the spinningsolution. The fiber which is formed is typically washed to remove anyadhering traces of the spin bath, and then dried.

In most cases, the solvent diluent which is employed provides thespinning composition (i.e., a spinning dope) with a room temperatureviscosity range between about 0.1-3,000 poises, and preferably betweenabout 100-1,000 poises.

Any useful solvent can be employed. Nonlimiting solvents include thosefor use with a water-miscible polymer and which include water and/orwater-miscible solvent such as methanol, ethanol, acetic acid,dimethylformamide, tetrahydrofuran, and the like. Solvents which can beused with an oil-soluble polymer include organic solvents such asbenzene, hexane, dichloroethylene, dichloroethylene, dimethylacetamide,dibutylether, ethylacetate, and the like.

The boron-containing polymers must be soluble in the solvents used todissolve the precarbonaceous polymer and form the spinning dope or atleast be soluble in solvents compatible with the precarbonaceous polymersolvents. It is preferred that the solvent for the boron-containingpolymer be the same as the solvent used to dissolve the precarbonaceouspolymer. It is not absolutely necessary that the solvent for theboron-containing polymer and the precarbonaceous polymer be the same aslong as the solvents are compatible. Compatibility as stated hereinmeans the solvents will form a homogenous mixture.

The concentrations of the polymeric materials in the spinning solutioncan vary widely and will depend for one on the particular spinningprocess, e.g., dry or wet which is used to form the fibers. Theconcentration of the boron-containing polymer is the controlling factorin solubility and, thus, for greater amounts of boron-containing polymerrequired, the solution will have to be less concentrated. Typically, forwet spinning, concentrations of the polymeric materials between about 5and 20% by weight will be used whereas for dry spinning, concentrationsof up to about 80% are useful. It is extremely difficult to obtainboron-containing polymer concentrations near 80% and, thus, for dryspinning, a much higher level of the precarbonaceous polymer relative tothe boron-containing polymer must be utilized. In such instance, theboron content of the formed fibers will be relatively low and, thus, dryspinning is not a preferred method of forming boron carbide fiberswherein the amount of boron relative to carbon must approach 3:1. On theother hand, the dry spinning process may be useful in forming boronnitride, boron phosphide or boron metalloid ceramic fibers inasmuch asthe amount of boron-containing polymer needed is the minimum to form anintact fiber. High levels of the precarbonaceous polymer do notadversely effect the non-carbide ceramic products since the polymer isburned away and is not present as a carbon source. The amount of theprecarbonaceous polymer therefore need not be controlled as in the caseof the boron carbide fibers. Preferably, wet spinning is used to formthe fibers since the greater amounts of solvent allow the use of agreater amount of boron containing polymer relative to theprecarbonaceous polymer.

After a newly formed fiber is spun, it can be stretched or drawn toabout 100-300% of its original length by conventional techniques.

The preceramic polymeric fiber can be converted to any one of a varietyof fibrous configurations prior to undergoing thermal treatment. Forexample, the fiber can be in the form of filaments, staple fibers, tows,plied yarns, knits, braids, fabrics, or other fibrous assemblages whileundergoing thermal treatment. Alternatively various fibrousconfigurations may be formed form the inorganic fibers at the conclusionof the pyrolysis step of the process.

To provide a final ceramic fiber product with optimal physicalproperties, it is preferred to subject the preceramic polymeric fiberfrom the preceramic fiber formation step to an initial thermal treatmentin a molecular oxygen environment. The polymers in the preceramic fiberare partially carbonized to a stabilized form so that the subsequentpyrolysis step of the process can be effected without the concomitantdestruction of the fibrous configuration. The thermal treatment step canbe conducted by heating the fiber in a molecular oxygen-containingatmosphere at a temperature ranging between about 200°-600° C. Thethermal treatment temperature selected is dependent upon the polymerresistance to distortion at elevated temperatures, and should not exceedthe polymer melting point during at least the initial phase of thethermal treatment.

Volatile components that evolve during the thermal treatment stepinclude water vapor and oxygen, and carbon monoxide and carbon dioxideresulting from a partial combustion of the polymers. Typically a 15-50%reduction in the weight of the fiber occurs during the thermal treatmentstep. It is believed that a crosslinking of carbon atoms occurs duringthe thermal treatment to produce a charred structure.

The thermal treatment can be performed in an autoclave by heating to therequired temperature/time schedule. A continuous thermal treatment canbe accomplished by the continuous passage of a fiber through a heatedchamber or calcining furnace. The fibrous structure of the fiber isretained throughout the thermal treatment step. There is a tendency forthe fiber to shrink while undergoing thermal treatment.

Alternatively, the preceramic fibers can be subjected to a chemicalstabilization treatment before being subjected to the pyrolysis step. Ina typical stabilization procedure, the dried fibers are contacted with areactive free radical-forming agent such as diazidoformamide, whicheffects the desired corsslinked structure in the fiber substrate atambient temperatures (e.g., 10°-40° C.).

In the subsequent pyrolysis step of the process, the preceramic fiber(either charred or uncharred) is subjected to a temperature betweenabout 700°-2,500° C. (preferably about 1,100°-1,800° C.). The pyrolysisperiod normally will range between about 0.2-8 hours. Any pyrolysis gascan be utilized to pyrolyze the fibers. Thus, inert gases will lead tothe formation of metal carbides while reactive gases including ammonia,phosphine, and metalloid-containing gases such as metal hydridesincluding germane, arsine, stibione, silane, etc. will lead to boronnitride, boron phosphides, and boron-metallic ceramics, respectively.Thus, if a carbide is desired, the pyrolyzation gas will be inert andthe precarbonaceous polymer will be one that does not easily burn awayso as to form a carbon structure which can be used for reaction. On theother hand, if the ceramic alloy is to be formed from reaction of theboron polymer and the pyrolyzing atmosphere, it may be desirable to useas the precarbonaceous polymer a polymer which burns off relativelyeasy.

The boron carbide fibers which are formed in accordance with the presentinvention have vastly increased levels of boron as compared with boroncarbide fibers produced by using boric acid as a boron source. Thus,typical boron loadings of boron carbide fibers using boron containingpolymers and, in particular, borane-containing polymers as the boronsource as in the process set out in the present invention will begreater than 10% by weight with substantially higher boron loadingsobtained depending upon the weight ratio of borane polymer toprecarbonaceous polymer contained in the spinning solution. Accordingly,boron levels as high as about 40 and even as high as about 50-80 wt. %are achievable by the process of the present invention.

EXAMPLE 1

Five grams of polyacrylonitrile were dissolved in 50 ml ofdimethylformamide by heating in water at about 50° C. Five grams ofdecarborane were added to the solution along with about 50 ml more ofdimethylformamide. Vigorous hydrogen gas evolution was observed. Thesolution was shaked occasionally until the gas evolution stopped. Thesolution had a yellow to amber color.

The solution was poured into water with stirring to precipitate awhite/yellow solid mass. Pockets of dimethylformamide which remainedlike blisters in the mass were punctured and the solid was collectedafter about five minutes in water. The mass was dried in an ovenovernight at 90° C. under vacuum. The mass was further dried at 90° C.for about three hours. The solid which remained was a brittle whitesolid which contained brown specks.

8.22 grams of the solid was ground and placed in a quartz tube. Amoderate argon flow was placed through the tube which was heated fromroom temperature to 1000° C. in about three hours and held at 1000° C.for another one half an hour. The argon flow was slowed and the tubecooled overnight. The material was in the form of a cylindrical, lightdensity black solid which weighed 5.34 grams, approximately 65% of thestarting 8.22 grams. The solid was analyzed and found to contain greaterthan 40% by weight boron.

EXAMPLE 2

Two spinning solutions to produce boron carbide fibers were formed withdecarborane as the boron source, polyacrylonitrile as the carbon sourceand dimethylformamide (DMF) as the solvent. The first spinning solutioncontained 1.5 grams of decaborane and 0.5 grams PAN in 12 ml DMF. Thesecond spinning solution contained 15 grams of decarborane, 15 grams ofPAN in 120 ml DMF. The respective spinning solutions were wet spunthrough a 70 micron aperture into a precipitating bath comprising 80%methanol and 20% dimethylacetamide at room temperature. The fibers werepyrolyzed in argon from room temperature to 1200° C. in three hours andmaintained at 1200° C. for two hours. The fibers were allowed to coolfor two days to room temperature. The pyrolyzed fibers were analyzed forboron content.

The first spinning solution containing a decaborane to PAN weight ratioof 3:1 yielded a boron carbide fiber containing 40% boron by weight. Thesecond sample containing a decaborane to PAN ratio of 1:1 yielded aboron carbide fiber containing 15% boron by weight. As can be seen, theweight ratio of boron source to carbon source is important in the finalboron content of the boron carbide containing fiber.

EXAMPLE 3

15.0 g decaborane are combined with 120 ml (113.3 g) DMF in a three-neckflask fitted with a mechanical stirrer. The solvent and decaborane arequickly deoxygenated and vigorously stirred for 2 hours. During thistime the initial bright yellow-orange color is replaced by a lessintense yellow while a white solid precipitates. Two equivalents ofhydrogen gas are given off during the exothermic reaction. As a safetyprecaution the reaction flask can be immersed in a cold water bathwithout appreciably prolonging the reaction time.

The white solid is the bis(dimethylformamidato) complex of decaborane.The contents in the flask are heated to 100° C. with vigorous stirringto redissolve the solid. The solution is an amber color. No gas isevolved during the heating which lasts approximately one-half hour. Thesolution is carefully cooled to room temperature and precautions aretaken to avoid exposing the solution while hot to oxygen. This solutionis a condensation polymer of decaborane and DMF. The polymer will notreprecipitate at room temperature at concentrations of less than 1M.

15.0 g of polyacrylonitrile (PAN) is added to the condensation polymersolution and, the solution thoroughly purged of oxygen. With vigorousstirring, the slurry is heated to 100° C. After the PAN dissolves, thepolymer mix is cooled.

A spinning dope is spun using a 100 micron jet and 5 lbs/in² N₂pressure. The extruded dope is coagulated in a four foot water bath,collected and dried in air.

The dried extrudate is pyrolyzed in ammonia under an ammonia gas flow of100 cc/min. The sample is brought from room temperature to 200° C. at arate of 6° C. per minute and held for one hour. The temperature is thenincreased to 1100° C. at a rate of 5° C./min. and held for two hoursbefore cooling overnight. The sample contains over 75 weight percentboron nitride.

What is claimed is:
 1. A preceramic blend comprising a mixture of aprecarbonaceous polymer and a preceramic boron-containing polymerwherein said boron-containing polymer consists essentially of thecondensation product of a borane with a Lewis base selected from thegroup consisting of amines, amides, isocyanates, nitriles andphosphines.
 2. The preceramic blend of claim 1, wherein said borane isdecaborane.
 3. The preceramic blend of claim 1 wherein the weight ratioof said boron-containing polymer to said precarbonaceous polymer is 9:1to 1:9.
 4. A preceramic fiber comprising a blend of a precarbonaceouspolymer and a preceramic boron-containing polymer wherein saidboron-containing polymer consists essentially of the condensationproduct of a borane with a Lewis base selected from the group consistingof amines, amides, isocyanates, nitriles and phosphines.
 5. Thepreceramic fiber of claim 4 wherein said precarbonaceous polymer ispolyacrylonitrile.